Try turning it on in "Safe Mode" and see if it will load. If it does, then you need to check the tablet for viruses, malware, bad apps, etc., remove the problem and then restart (turn off and back on) the device.
If, however, it won't start up even in "Safe Mode" then you are either looking at a "Hard Reset", which will wipe the device of all data and start it over clear of all your stuff, or taking it to a tech to look at (who still may do the "Hard Reset").
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.